Compressors exist of a series of rotating or stationary blade rows in which the combination of a rotor (circular rotating blade row) and a stator (circular stationary blade row) forms one stage. Inside the rotor, kinetic energy is transferred to the gas flow (usually air) by the individual airfoil blades. In the following stator, this energy is manifested as a pressure rise in the gaseous air as a consequence of deceleration of the gaseous air flow. This deceleration of the gaseous air flow is induced as a result of the design of the stator section. The pressure ratio (exit pressure/inlet pressure) of a single stage is limited because of intrinsic aerodynamic factors, so several stages are connected together in many turbo compressors to achieve higher pressure ratios than could be achieved by a single stage.
When operating an axial compressor, the problem of "fouling" arises, namely the continuous contamination of the surfaces of the rotor and stator blades with oil and dust, particularly of the first compressor stages at the inlet of the compressor.
In an initial phase of the fouling problem, an increased surface roughness of the blades may be observed, influencing the behavior of the boundary layer air associated with the blades. The air fluid flow around each blade has an associated flow boundary layer which covers each blade and coheres to the blade. The flow boundary layer associated with a rotor blade will rotate as an associated entity of the blade as the blade itself rotates. At the downstream edge of each blade, this flow boundary layer melds into an associated flow boundary entity known as, alternatively, the Dellenregion, wake region, or delve region which is characterized by a localized reduction in both pressure and flow velocity. As in the case of the flow boundary layer, the associated wake region rotates with its rotor blade. As the contamination collects on the blade over a period of time, the resultant surface roughness also increases, which causes the thickness of the flow boundary layer also to increase. As a result, the wake region becomes more and more extensive and pronounced. Therefore, increasing thickness of the boundary layer will produce higher losses of the total pressure throughout the bladings, leading to efficiency reduction of the compressor.
Axial compressors therefore are "washed" after certain operation intervals by cleaning the blades of at least the front stages.
The time interval between successive cleaning operation should not be too long, as otherwise the compressor is operated with too much reduced efficiency. Under certain circumstances, the danger of compressor stall or compressor surge increases. Due to the reduced efficiency, the compressor load has to be increased (operating point moves closer to stability line) to maintain the outlet pressure.
On the other hand, it is quite uneconomical when the compressor is cleaned after a relatively short operation period, particularly as the cleaning leads to a distinct operational interruption.
Therefore, it is desirable to measure the actual fouling state of the compressor, in order to determine the optimum time for said "washing".
Contemporary turbo engines are usually equipped with fuel or energy control systems which measure and output a variety of operating parameters for the overall engine. Included in such control systems are highly accurate pressure sensing devices or systems. For example, pressure measuring systems are described in U.S. Pat. No. 4,322,977 entitled "Pressure Measuring System" filed May 27, 1980 in the names of Robert C. Sell, et al; U.S. Pat. No. 4,434,664 issued Mar. 6, 1984, entitled "Pressure Ratio Measurement System", in the names of Frank J. Antonazzi, et al.; U.S. Pat. No. 4,422,335 issued Dec. 27, 1983, entitled "Pressure Transducer" to Ohnesorge, et al.; U.S. Pat. No. 4,449,409, issued May 22, 1984, entitled "Pressure Measurement System With A Constant Settlement Time" in the name of Frank J. Antonazzi; U.S. Pat. No. 4,457,179, issued Jul. 3, 1984, entitled "Differential Pressure Measuring System", in the names of [J Bluish] Frank J. Antonazzi, et al.; and U.S. Pat. No. 4,422,125 issued Dec. 20, 1983, entitled " Pressure Transducer With An Invariable Reference Capacitor", in the names of Frank J. Antonazzi, et al.
While a wide variety of pressure measuring devices can be used in conjunction with the present invention, the disclosures of the above-identified patents and the article mentioned next are hereby expressly incorporated by reference herein for a full and complete understanding of the operation of the invention.
As initially mentioned, the efficiency of said axial compressor depends on the fouling state thereof. On line derivation of the efficiency of said compressor, however, is only an indirect indicator of the fouling status and cannot be used to derive direct conclusions concerning the state of fouling; there also are many other parameters influencing the characteristics of the flowing air in the high pressure stages of said compressor and the efficiency of those stages, and those other parameters are difficult to measure.
The article "Fast Response Wall Pressure Measurement as a Means of Gas Turbine Blade Fault Identification" by K. Mathioudakis et al as presented at the "Gas Turbine and Aeroengine Congress and Exposition" from Jun. 11-19, 1990, Brussels, Belgium, ASME Paper No. 90-GT-341, relates to the identification of blade faults. For simulating fouling of the rotor, all blades of one rotor of the compressor were coated with a textured paint, said paint layer roughening the surface and causing a slight alteration of the contour of the blades. The dynamic pressure field around the rotor was measured by fast response pressure transducers at the inner circumferential surface of the rotor housing. From the time depending pressure sensor signals, respective frequency signals (power spectra) were derived and compared with respective frequency signals of an intact compressor (without blade painting). Respective tests were performed for other blade faults, as there are bended or twisted blades, rotors with only two faulty blades (simulated by painting only these two blades). The latter faults could be more or less clearly identified from comparison of the respective power spectra. To this aim, certain indices were derived from the power spectra to be compared, namely the ratio of spectra amplitudes and the logarithm thereof. These tests show that a discrimination of the mentioned test faults is principally possible, for which complex simulation- calculations have to be performed.
This article does not deal with the problem of an actual determination of one blade fault, namely of blade fouling. The experimental setup with painting of all rotor blades is only a rough simulation of the actual fouling process, which is characterized by a more subtle increase of surface roughness of the blades during the operation time of the compressor.